1. Field of the Invention
The invention is directed to a method and a circuit for processing the signals of an earth-horizon sensor in a circularly orbiting earth satellite, wherein said earth-horizon sensor operates in the infrared range and includes, in the focal plane of an input lens, a chopper disk which is periodically reciprocated with a determined chopper frequency and which has a diameter corresponding to the image of the earth, and a detector receiving periodically interrupted light radiations of oppositely located earth rims and wherein the output signal of the detector is synchronously demodulated with the chopper frequency in order to derive an off-course signal.
2. Description of Related Art
For the preparation of attitude control signals of such, e.g. geostationary earth satellites, optical earth-horizon sensors, which can be classed with the zero-seeking sensors, are used for two axes. Such an earth-horizon sensor operates in the infrared range and is based on the mechanical vibrator or chopper principle. The infrared radiation of the earth is collected by means of an objective lens of germanium and falls on a circular chopper disk in the focal plane of the lens. This chopper disk has a diameter corresponding approximately to the image of the earth and is periodically reciprocated with a determined chopper amplitude and a determined chopper frequency. The light of the two oppositely located earth rims which are alternately released with the chopper frequency, the light being periodically interrupted by the chopper disk, is guided, via a spectral filter for the infrared range, onto a detector, e.g. a pyroelectric detector, via a secondary lens consisting of a spherical mirror segment and a prism. The oscillation of the chopper disk is sine-shaped; the chopper amplitude is stabilized by means of an electronic drive unit.
When the optical line of sight of the earth-horizon sensor is directed to the earth central point, the oppositely located earth rims are periodically pictured on the detector which produces, after amplification, an electric signal similar to that of a rectified sine wave. If the radiation portions of the oppositely located earth rims are identical, then the amplified signal is transformed in a synchronous demodulator, into a sine oscillation without a direct current component, wherein the synchronous demodulator is keyed with the chopper frequency so as to be rigidly in phase. The zero signal obtained in this way determines the zero point of the off-course signal used for the attitude control of the earth satellite.
If the line of sight of the earth-horizon sensor is not directed to the earth central point, then unequal radiation portions from the opposite earth rims impinge on the detector. The detector again supplies a signal in the form of a rectified sine-wave voltage, but the amplitudes of the half-waves are different. The synchronous demodulator, which is again keyed with the chopper frequency so as to be rigidly in phase, then generates a direct current voltage which depends, approximately linearly, on the deviation of the line of sight of the earth-horizon sensor from the satellite/earth central point connecting line. This off-course signal can then be used for the attitude control.
Up to this point it has been assumed that the two oppositely located earth rims have the same temperature; as a rule, however, this is not the case. Particularly in the north-south direction of the earth, the oppositely located earth rims have different temperatures, which is commonly designated as earth anomaly, by means of which the demodulated off-course signal has a direct current component also when the optical line of sight of the earth-horizon sensor is actually directed toward the earth central point. The demodulated off-course signal, accordingly, still contains an anomaly portion caused by the earth anomaly, which anomaly portion can be described by means of a zero point displacement of the characteristic line of the earth-horizon sensor in the ordinate direction, wherein the sensor characteristic line is displaced in the direction of the side with the weaker radiation, i.e. the radiation assigned to a lower temperature. Therefore, care must be taken that this anomaly portion is corrected. This anomaly error within the linear measuring area of the earth-horizon sensor of approximately .+-.1.degree. is only slight, but can amount, in extreme cases, to approximately .+-.16% of the linear measuring range. Since geostationary earth satellites are also used for producing directional radio links and guided television links to the earth, the anomaly portion must be suppressed as much as possible.